Embodiments relate to air handling units and, more particularly, to methods and systems for active sound attenuation in a fan unit, which may be used in an air handling system, for example.
Air-handling systems (also referred to as air handlers) have traditionally been used to condition buildings or rooms (hereinafter referred to as “structures”). An air-handling system may contain various components such as cooling coils, heating coils, filters, humidifiers, fans, sound attenuators, controls, and other devices functioning to at least meet a specified air capacity which may represent all or only a portion of a total air handling requirement of the structure. The air-handling system may be manufactured in a factory and brought to the structure to be installed or it may be built on site using the appropriate devices to meet the specified air capacity. The air-handling compartment of the air-handling system includes the fan inlet cone and the discharge plenum. Within the air-handling compartment is situated the fan unit including an inlet cone, a fan, a motor, fan frame, and any appurtenance associated with the function of the fan (e.g. dampers, controls, settling means, and associated cabinetry). The fan includes a fan wheel having at least one blade. The fan wheel has a fan wheel diameter that is measured from one side of the outer periphery of the fan wheel to the opposite side of the outer periphery of the fan wheel. The dimensions of the air handling compartment such as height, width, and airway length are determined by consulting fan manufacturers data for the type of fan selected.
During operation, each fan unit produces sounds at many frequencies. In particular, smaller fan units typically emit sound at higher audible frequencies, whereas larger fan units emit more energy at lower audible frequencies. Devices have been proposed in the past that afford passive sound attenuation such as with acoustic absorption or sound barriers that block or reduce noise transmission. Acoustic absorption devices include a soft surface that converts sound energy to heat as the sound wave is reflected within the fan unit.
Some fan units are configured to control inlet noise through the use of sound traps located upstream of the fan. The sound traps may be located either in ductwork or in a special inlet section of an air handler enclosure. However, the sound traps typically occupy significant space in the ductwork or air handler enclosure. Moreover, the sound traps typically add significant cost to the fan units. Further, the sound traps typically do not provide for attenuation targeted at specific tonal nodes.
A need remains for improved systems and methods to provide sound attenuation in air handling systems.